The invention relates to bearing assemblies.
In designing a rotor system comprising a rotary shaft held in bearings mounted on a frame, one consideration is for the frame to be able to support the bearings in correct alignment with the shaft upon its rotation at operational speeds. Errors in the construction or assembly of the rotor system, or thermal or other distortions of the frame, in operation, might cause the bearings to become dangerously misaligned with the shaft if the bearings were held rigidly with respect to the frame. It is known in the art to mount journal and thrust bearings in spherical shells so that they have some degree of freedom to align themselves with their shafts in the event of distortion of the frame, such bearings being referred to as xe2x80x9cself-aligning bearingsxe2x80x9d. In some circumstances, however, such spherical shells are not an attractive means of providing self-alignment.
The invention provides a bearing assembly for supporting or locating a rotary shaft in a frame, the assembly comprising an outer shell for fixing relative to the frame, the outer shell providing a pair of coaxial annular surfaces spaced apart and facing inwardly towards each other, and an inner shell within the outer shell, the inner shell providing a pair of coaxial annular surfaces, each of which faces outwardly towards one of the inwardly-facing annular surfaces of the outer shell and is arranged substantially coaxially with, and adjacent to, its associated inwardly-facing annular surface, the inner shell being able to move axially relative to and to tilt relative to the outer shell and to move axially relative to and to tilt relative to the shaft, and each of said inwardly-facing annular surfaces and/or its associated outwardly-facing annular surface having one or more channels arranged to extend around the surface, the bearing assembly also comprising means for supplying a fluid from a high pressure source to each of the said channels through flow resistance means.
With such a bearing assembly, the inner shell may be arranged as a journal bearing or as a thrust bearing for a rotary shaft.
In operation, the outer shell of the bearing assembly is fixed relative to a frame provided to support the rotary shaft. The inner shell holds the rotary shaft in its journal or thrust bearing. Fluid from the high pressure source is introduced through the flow resistance means, which is preferably one or more entry passages of restricted cross-sectional area for each channel, into the channels of the annular surfaces. The fluid flows around the channels and leaves the channels to flow radially inwards and outwards through gaps between the associated inwardly- and outwardly-facing annular surfaces of the shells, from where it escapes to the surroundings outside the outer shell at ambient pressure. The channels are generally of such radial width and depth, and the gaps between each inwardly-facing annular surface and its associated outwardly-facing annular surface are sufficiently narrow, that the resistance to the flow of fluid around the channels is small compared to the resistance to flow through the gaps between the annular surfaces of the inner and outer shells. Consequently, the pressure exerted by the fluid around each channel is effectively constant and is determined by the width of the associated gap, increasing as the width of the gap decreases, and decreasing as the width of the associated gap increases.
Accordingly, movement of the inner shell in one direction along the axis relative to the outer shell will cause the pressure exerted by the fluid in the channel or channels to increase on the side of the inner shell on which its. spacing with the outer shell has decreased and to decrease in the channel or channels on the other side of the inner shell where the spacing with the outer shell has increased. The pressure difference across the inner shell creates a restoring force on it urging it back to an equilibrium position, for example, a central position within the outer shell. In such an arrangement, the restoring force exerted on the inner shell can provide substantial resistance to displacement of the inner shell in an axial direction relative to the outer shell.
The channel or channels in each of the said inwardly-facing annular surfaces and/or its associated outwardly-facing annular surface advantageously extend substantially entirely around the surface, and preferably extend substantially concentrically around the surface. Advantageously, each of the said inwardly-facing annular surfaces and/or its associated outwardly-facing annular surface has a continuous channel. With such an arrangement, the pressure distribution around each of the said annular surfaces at a given radius is substantially uniform, in operation. The mean radius of such a channel is preferably substantially equal to the square root of the product of the inner and outer radii of the annular surface in which it is located. The channel in each surface need not, however, be a continuous channel, and there may be two or more arcuate channels of equal mean radius, or even two or more channels of different mean radii in each of the said annular surfaces, and the configuration of the channel or channels in one annular surface need not be the same as that of another, but the pressure distribution around each annular surface at a given radius is preferably substantially uniform, in operation.
When the bearing assembly is arranged to provide a journal bearing, the inner shell advantageously comprises a journal bearing through which the shaft, or journal, can extend, and an annular flange on the journal bearing, which provides the said outwardly-facing annular surfaces. Such an arrangement can, by exerting a substantial restoring force on the flange of the inner shell should it move in an axial direction from its predetermined position relative to the outer shell, provide positive location of the journal bearing relative to the apparatus frame while allowing inclination of the flange of the inner shell relative to the outer shell and, hence, self-alignment of the journal bearing with the rotary shaft. The extent to which the flange of the inner shell can become inclined to the outer shell, and hence the limits of the self-aligning capability of the journal bearing, is determined by the dimensions of the inner and outer shells and the sizes of the gaps between the annular surfaces.
In an arrangement in which continuous channels are provided in the annular surfaces, although it is possible to arrange for the bearing assembly to provide a substantial restoring force against relative displacement of the inner shell in an axial direction as discussed above, if associated annular surfaces become slightly inclined to each other the pressure will still be substantially uniform at a given radius around each surface and no significant restoring moment against that inclination will be created. Thus, in the event of inclination of the inner shell with respect to the outer shell, as, for example, on self-alignment of a journal bearing as referred to above, the inner shell will remain inclined to the outer shell. Such an arrangement is suitable for journal bearings having self-generating capabilities, that is to say, the capability, on rotation of the shaft at a rate of rotation above a threshold rate, of generating a continuous film of lubricant fluid between the bearing and the shaft to prevent solid to solid contact. Examples of suitable self-generating journal bearings are a simple self-generating journal bearing of circular bore, a tilting-pad journal bearing, a journal bearing with fixed lands, or a journal bearing as described and claimed in our co-pending patent application No. (case II) filed on the same date as this patent application.
When the bearing assembly is arranged to provide a thrust bearing, the inner shell advantageously provides inwardly-facing thrust and surge bearing surfaces for acting on each side of a thrust collar of a rotary shaft. With such an arrangement, thrust exerted by the thrust collar on the thrust bearing surface of the inner shell such as to cause axial displacement of the inner shell is resisted by the restoring force created when the inner shell moves from its equilibrium position within the outer shell. Advantageously, the inner shell provides two inwardly-facing co-axial annular surfaces, each of the said inwardly-facing annular surfaces of the inner shell having one or more channels arranged to extend, preferably, substantially concentrically, around the surface, and there is also provided means for supplying a fluid from a high pressure source to each of the said channels through flow resistance means, for example, one or more entry passages of restricted cross-sectional area for each channel. The said inwardly-facing annular surfaces of the inner shell form the thrust and surge bearing surfaces, respectively, of the thrust bearing. The outwardly-facing and inwardly-facing annular surfaces of the inner shell are advantageously co-axial, and are preferably provided by annular members spaced apart from each other in an axial direction. With such an arrangement, in operation, the annular members of the inner shell are located on each side of an annular thrust collar of the rotary shaft, the inwardly-facing surfaces of the inner shell being arranged adjacent to, but separated from, the annular surfaces of the thrust collar to allow relative movement between the thrust collar and the inner shell in an axial direction and also to allow inclination of the inner shell relative to the thrust collar.
If the said inwardly-facing annular surfaces of the inner shell are each provided with a continuous channel, then, in addition to the restoring force acting on the inner shell arising as a result of axial displacement of the inner shell relative to the outer shell as explained above, an additional restoring force is created between the inner shell and the thrust collar in the event of relative axial displacement that acts to keep the inner shell at an equilibrium position relative to the thrust collar. Hence it is possible to arrange for the bearing assembly to provide a substantial restoring force against thrust transmitted by the thrust collar. If, however, each of the said inwardly-facing annular surfaces of the inner shell is formed with two or more arcuate channels of substantially equal mean radius instead of a continuous channel, and each arcuate channel is supplied with fluid from a high pressure source independently through its own entry passage then, although the pressure exerted by the fluid within each channel will be substantially constant, and the pressure exerted in the channels of each annular surface can be substantially equal when that annular surface is in a parallel plane to that of the adjacent annular surface of the thrust collar, the pressures exerted in the channels of the same annular surface will change relative to each other as that annular surface is inclined relative to the annular surface of the thrust collar at least in one sense. Thus, it is possible to arrange for a substantial restoring moment to arise between the inner shell and the thrust collar in addition to the restoring force acting between the two. In order to produce a restoring moment about two axes perpendicular to each other and to the axis of the shaft, as is advantageous, three or more arcuate channels, preferably of substantially equal mean radius, and desirably of substantially equal length, should be provided on each of the inwardly-facing annular surfaces of the inner shell. With such an arrangement, the inner shell provides a thrust bearing able to align itself with the thrust collar of a rotary shaft in the event of relative inclination of the outer shell to the thrust collar caused, for example, by distortion of the frame. With an arrangement in which there is substantially no restoring moment between the outer shell and the inner shell, inclination of those surfaces to each other is maintained, but the restoring force between the outer shell and the inner shell remains the same upon relative movement in an axial direction between the inner and outer shells. Such an arrangement allows relative inclination to occur between stationary surfaces (that is to say, surfaces that do not rotate with the shaft) of the inner and outer shells whilst tending to maintain alignment between the inner shell and the thrust collar, which are in relative movement, reducing the risk of solid to solid contact between stationary and rotating surfaces.
Such an arrangement of bearing assembly is suitable for thrust bearings of a self-generating kind such as tilting pad thrust bearings, or thrust bearings that combine self-generation with aerostatic or hydrostatic separation, for example, by providing annular surfaces of the thrust collar with spiral channels handed to diminish by their pumping action the radial flow of fluid in the gaps between the thrust collar and the inner shell.
The fluid introduced into the channels from the high pressure source may be air, and is advantageously air in some applications, but the fluid may be another gas, or it may also be a liquid, for example, water.
In practice, the extent to which the inner shell can move axially within the outer shell, or become inclined to the outer shell, is relatively small, the gaps between adjacent annular surfaces being relatively fine.
The configuration and dimensions of the inner and outer shells including the channels in the annular surfaces, the width of the gaps between the annular surfaces, and the size of the flow resistances is a matter of detailed design in any particular case but can readily be determined in practice for any particular application.